1. Field
The present disclosure relates generally to telecommunications, and more particularly, to various concepts and techniques for handover in a wireless communications system.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. The networks may be multiple access networks capable of supporting communications for multiple users by sharing the available network resources. An example of such a network is a Universal Terrestrial Radio Access Network (UTRAN). UTRAN is the Radio Access Network (RAN) that is part of the Universal Mobile Telecommunications System (UTMS), a third generation (3G) mobile phone technology promulgated by the “3rd Generation Partnership Project” (3GPP). UMTS, which is the successor to Global System for Mobile Communications (GSM), currently uses Wideband Code Division Multiple Access (W-CDMA) as the underlying air interface in the UTRAN architecture with the existing GSM infrastructures for the core network.
In the UTRAN architecture, the RAN is divided into a number of Radio Network Subsystems (RNS), each controlled by a Radio Network Controller (RNC). The RNC is a node responsible for, among other things, assigning, reconfiguring and releasing radio resources within the RNS. The RNCs may be interconnected through an interface comprising a direct physical connections or virtual networks using any suitable transport network.
The geographic region covered by a RNS is divided into a number of cells, with a base station serving each cell. A base station, which is referred to as a Node-B, is a node having a radio transceiver to support an air interface with all wireless terminals in its cellular region. A wireless terminal, which is referred to as “user equipment” (UE), uses a Radio Resource Control (RRC) protocol to obtain radio resources. RRC is a link layer protocol which is terminated at the RNC. Below the RRC layer in the protocol stack is another link layer protocol known as Radio Link Control (RLC). The RLC layer, which is terminated at the Node-B, provides for the retransmission of data and controls to achieve a lower error rate than the physical layer could achieve alone.
As the UE roams from one cell to another in a RAN, various handover procedures are implemented to ensure that the UE maintains its connection with the core network. A handover is a process in which the RAN changes the radio transceivers to provide bearer services maintaining a defined service level. Handover may be initiated by the UE or the RAN based on transmission criteria (e.g., signal strength, signal quality, power level, propagation delay, etc.) as well as traffic criteria (e.g., load balancing, backhaul constraints, maintenance, etc.).
A RNS handover procedure involves tearing down the RLC connection with a serving Node-B and reestablishing it with a target Node-B. The procedure also requires transferring the RRC context (i.e., RRC connection) between the serving and target RAN. A problem arises when RRC messages are lost in transet as the RLC connection is being torn down and reestablished. This tends to result in dropped calls. Because the handover of the wireless terminal between RNSs is far less frequent than the handover between cells within a RNS, the level of dropped calls have not gained much attention.
Evolved UTRAN (E-UTRAN) is 3GPP's proposal of an evolution of the 3G W-CDMA system. An E-UTRAN architecture includes Evolved Node-Bs (eNode-B) dispersed through the RNS to support an air interface with wireless terminals. The RNCs have been replaced with Evolved Packet Cores (EPC) and moved from the RAN to the core network. As a result, the RRC layer has been pushed down into the eNode-B. With the RRC terminated at the eNode-B, the frequency of dropped calls from lost RRC messages will likely increase. Accordingly, there is a need in the art for a more robust handover procedure in E-UTRAN architectures. The solution should be generic enough to be extended to other network architectures.